In some power transmission systems, fuses are used for protection on the high side (primary) of distribution transformer banks. Such fuse protection is frequently found, for instance, in delta-wye connected distribution transformer banks, particularly in those situations where the distribution load does not warrant the cost of installing circuit breakers on the high side of the transformer banks. When the fuse(s) for one or more of the three phases on the high side of the distribution transformer operates, i.e. blows, unbalanced voltages are applied to the transformer bank as well as to the load which is connected to the low side (secondary) of the transformer bank. With respect to the load, which may for example be an induction motor, the unbalanced voltage will cause a significant amount of negative sequence current to flow in the load, which may damage the load if it is present for too long a time period. In addition, the resulting circulating current in the delta-wye connected transformer bank caused by the unbalanced voltages will heat the distribution transformer bank, perhaps to the point where the transformer bank may be lost, which is expensive and time-consuming to replace.
High side transformer fuses, while usually reliable, can fail for a variety of reasons, including a specific fault condition, or they can be progressively weakened by previous events which are insufficient to cause them to operate and then fail later spontaneously. In any event, the consequences of a high side fuse operating (blowing) are potentially severe, both to the load and to the transformer bank itself, regardless of the cause of the operation. Hence, it is desirable to develop a simple, low cost detection system for detecting high side fuse operations which can then be used to trip distribution breakers or a low side bank breaker to protect the transformer and/or the load. It would be particularly desirable if such a detection system could be incorporated in the protective relays which are used to protect the distribution lines on the low side of the transformer bank.